1. Field of the Invention
The present invention relates to the crosslinking of polysilazane polymers and to the use of such crosslinked polysilazanes for the production of ceramic materials based on silicon nitride and/or silicon carbide.
2. Description of the Prior Art
The production of reinforced or unreinforced ceramic materials based on silicon carbide and/or silicon nitride by the thermal decomposition of precursor compounds of the polysilazane type, in particular organopolysilazanes or organopoly(disilyl)silazanes, is known to this art and is widely described in numerous articles and in the patent literature.
The organopolysilazanes and organopoly(disilyl)silazanes, hereinafter designated the polysilazanes, are well known products and exist in the form of monomers, oligomers and cyclic or linear polymers, and also in the form of polymer resins. These polysilazanes may be prepared by a wide variety of processes from a wide range of starting materials.
One advantage of a "polymer" method for the preparation thereof resides in the option of shaping this type of product into fibers or coatings of varying thicknesses, and ultimately producing, after pyrolysis, ceramic articles of Si.sub.3 N.sub.4, SiC, SiNC, or mixtures thereof.
Indeed, the ability to shape the polysilazanes into the form of fibers is probably one of their principal advantages.
According to a method which has now become conventional, the polysilazanes (after softening or melting, if originally they are in the solid state) are processed into continuous filaments by any known means, in particular by extrusion through dies, then pyrolyzed to improve their heat resistance and mechanical strength and, ultimately, to provide fibers or filaments of the desired ceramic material
The preliminary treatment of the fibers prior to pyrolysis, typically designated hardening, infusibility treatment or crosslinking, constitutes an essential stage in any process for the preparation of ceramic fibers via silazane polymers.
Heretofore, the hardening of polysilazane fibers was carried out either by physical methods (e.g., irradiation) or thermochemical methods.
The principal disadvantages of the physical methods are that they are delicate and expensive and the only method currently being carried out on an industrial level is thermochemical hardening by means of a treatment under a mixture of air and water vapor.
However, such a treatment presents the severe disadvantage of introducing large amounts of oxygen into the ceramic fiber, which may give rise to appreciable changes in the performance of such fibers, in particular relative to thermomechanical properties, such as, for example, their rupture strength.